The present invention relates to core-shell polymer nano-particles and method for preparing them. The particles can include a crosslinked core and a crosslinked shell, where the shell is grafted to the core through linkage with residual unsaturation carried in the core. The core-shell polymer nano-particles provide a controlled structure, size, and morphology which are useful in composite materials, such as for example, as fillers in elastomeric polymers.
Incorporation of particles as fillers in rubber compositions is common, especially in the tire arts. The fillers are chosen to enhance certain characteristics of the rubber composition. The use of fillers, such as carbon black, silica, and crosslinked polymers, is well known. The addition of fillers tends to improve the physical properties of the polymer matrices to which they are added. Some specific physical properties that are improved by the addition of fillers are tensile strength and wear.
U.S. Pat. Nos. 5,395,891 and 6,127,488 to Obrecht disclose the use of crosslinked polybutadiene and crosslinked styrene-butadiene copolymers respectively as fillers. The purported benefits of the inclusion of these crosslinked polymer particles is lower hysteresis while the polybutadiene gels also impart improved abrasion resistance and the styrene-butadiene copolymer gels offer improve wet traction. U.S. Pat. Nos. 6,133,364; 6,207,757; and 6,242,534 to Obrecht et al. disclose a rubber composition comprising a crosslinked elastomer particle and a non-crosslinked elastomer. The crosslinked elastomeric particles are functionalized to impart unique surface reactivity for improved coupling with the non-crosslinked host elastomer as well as the subsequent ability to employ coupling agents other than the conventional sulfur-bridged bis-alkoxy silanes. The benefits of the gels are reported to be lower hysteresis, improved abrasion resistance, and improved wet traction.
U.S Patent Applications published as US 2002/0007011 and US 2001/0053813 to Konno et al. disclose a rubber composition comprising as essential components a crosslinked polymer particle and a non-crosslinked rubber component. The rubber composition may be used to obtain a vulcanized rubber. The crosslinked polymer particles are intended for use as filler to provide a rubber composition having good processability and handling performance as well as improved tensile strength and wearing resistance when vulcanized. However, it has been found that the particles disclosed in the Konno et al. application tend to soften at higher service temperatures. The effectiveness of the particles as reinforcing fillers is therefore decreased.
Core-shell morphologies from polybutadiene-polystyrene graft copolymers are taught by U.S. Pat. Nos. 6,248,419 and 6,489,378 to Sosa et al. But, the teaching is directed to production of high impact polystyrene using graft copolymer particles having a polystyrene core occluded inside a polybutadiene shell and the particles have an average particle size of about 0.5 microns.
Nano-particles are currently receiving significant interest as fillers in rubber compositions. Such combinations often provide dimensions and physical properties that differ from those of the bulk material. The use of nano-particles as fillers is thought to be advantageous because the discrete particles may be more easily and better dispersed in the polymer matrix. The higher surface to volume ratio of the nano-particles provides more opportunities for chemical and physical interactions with the polymer matrix. For example, U.S. Pat. No. 6,437,050 to Krom et al teaches a polymer nano-particle composition wherein the core is a poly(alkenylbenzene) and the surface layer includes a poly(conjugated diene) and wherein the particles are made by a dispersion polymerization process. The particles disclosed in the Krom patent are described as having core-shell morphology, but it is believed that the consensus would describe the particles as star block copolymers, as is discussed and taught in xe2x80x9cSynthesis and Properties of Block Copolymers. 3. Polystyrene Polydiene Star Block Copolymers,xe2x80x9d Bi, L.; Fetters, l.j.; Macromolecules, 1976, Vol.9, No.5, pg. 732-742. Each of these processes uses a solvent polymerization technique.
Nano-particles have also been made using seeded emulsion polymerization technique. Ferguson et al, Polymer magazine, Vol. 43, Issue 24 (November 2002), pages 6371-6382 discloses a process in which particles containing a polystyrene core may be synthesized with a poly(vinyl acetate) shell by emulsion polymerization without secondary particle nucleation. Ferguson et al focuses on the synthesis of particles with diameters of about 400 nm and only briefly discusses the synthesis of polymer particles with diameters less than 100 nm. Ferguson et al teaches that the emulsion polymerization technique used may produce polystyrene core/poly(vinyl acetate) shell polymer particles where most of the seed particles have diameters of more than 200 nm and reported effort apparently is to achieve particles with diameter of more than 200 nm.
The present invention is the result of the discovery that polymer particles with a core-shell morphology can be prepared from aromatic vinyl monomers and conjugated diene monomers by an emulsion polymerization technique, and the resulting core-shell polymer nano-particles provide a controlled structure, size, and morphology which are very useful, for example, as fillers in elastomeric polymers to enhance their properties. The particles can make compatible two otherwise immiscible polymers by coating one of the polymers (as the core) in a polymeric shell. The shell is miscible with the polymer matrix, or host polymer, in which the particles are ultimately dispersed.
The core and the shell can be varied depending upon the desired properties and use of the particles. The core can be a soft or hard polymeric composition. The core can be a crosslinked polymer having a crosslinked shell. The shell can be grafted onto the core through linkage with residual unreacted vinyl groups carried in the core. The volume fraction of the core and shell can be varied depending upon the desired final properties of the particles. The shell composition can be chosen to provide compatibility with the matrix composition. The core-shell particle can comprise a crosslinked polymer core where the crosslinked polymer core has a predetermined Tg such that the crosslinked polymer core does not soften at service temperatures up to about 150xc2x0 C.
The particles are synthesized by an emulsion polymerization which can be a batch, semi-batch, or seeded continuous process, but which provides excellent control of the polymer composition and morphology. The process produces particles having a mean average diameter of less than 200 nm, preferably less than about 100 nm. The core-shell particles are useful as a filler in polymer compositions and the particles of the present invention provide enhanced properties for the polymer compositions.